1. Field of the Invention
The present invention relates to a novel aluminum nitride sintered body and a method of producing the same. More specifically, the invention relates to an aluminum nitride sintered body comprising crystal grains of relatively small grain sizes, containing pores in very small amounts, suited particularly as a substrate to be mirror polishing and having a large strength.
2. Description of the Related Art
Owing to their excellent thermal conducting properties, insulating property and free of toxicity, aluminum nitride sintered bodies have been put into practical use as various substrate materials having heat-radiating function.
The aluminum nitride sintered bodies have heretofore been produced on an industrial scale by sintering an aluminum nitride powder to which a sintering aid is added at a temperature of as high as 1700° C. or higher (hereinafter referred to as resistance heating sintering method).
The resistance heating sintering method is the one by which the sintering is effected over extended periods of time during which crystal grains of aluminum nitride grow to constitute the sintered body. Therefore, the sintered body obtained by the above method has a relatively large crystal grain size. Further, a gas entrapped in the starting aluminum nitride powder grows together with the growth of crystal grains developing a phenomenon of forming relatively large pores.
Therefore, when it is attempted to machine the surface of the sintered body obtained by the above resistance heating sintering method like a mirror surface to use it as a substrate for a circuit, large grain size and the presence of pores make it difficult to form highly fine wiring patterns. Besides, the sintered body has a small strength due, presumably, to the crystal grain size and the presence of pores leaving room for improvement even from the standpoint of strength.
In producing a ceramic sintered body, on the other hand, there has been known a method of decreasing the crystal grain size of the obtained sintered body by exposing the aluminum nitride powder to which a sintering aid is added into a plasma (patent document 1: JP-A-2-172869).
The above method may make it possible to obtain a sintered body having small crystal grain sizes. In conducting the sintering, however, infiltration of a plasma gas into the interior is an essential requirement. Besides, the sintering completes in short periods of time and, hence, the gas tends to be trapped in the sintered body. As a result, though the method of the patent document 1 makes it possible to achieve a sintering density which is high to some extent, pores unavoidably remain in the sintered body. Presence of pores makes it difficult to use the sintered body as a substrate for circuits that require the formation of highly fine wiring patterns. Besides, this method requires a facility of a large scale for generating and sustaining a plasma, involving a problem in that it is not easy to put the method into practice on an industrial scale.
In recent years, on the other hand, there has been proposed a method of producing a ceramic sintered body by utilizing a so-called “discharge plasma” by feeding a pulse current while pressing a ceramic powder. Concretely speaking, there has been proposed a method of sintering the ceramic powder in a mold by the action of a plasma that is presumed to take place in the ceramic powder by filling the molding die with the ceramic powder under a pressurized state between the electrodes and by heating the molding die by feeding a pulse current (patent document 2: JP-A-2000-128648).
The above sintering method utilizing the discharge plasma is conducted by using a simple facility while elevating the temperature by heating the molding die. Therefore, the temperature can be easily controlled and, hence, attention has been given to this sintering technology as a future technology. According to this method, further, the ceramic powder is heated to a sintering temperature within a short period of time, the grains are suppressed from growing during the sintering, and it is expected that there can be obtained a sintered body having a mechanical strength greater than that obtained by the conventional resistance heating sintering method.
However, when the above discharge plasma is applied to sintering the aluminum nitride powder, it was found that pores are formed in large amounts though the sizes thereof are smaller than those formed by the resistance heating sintering method. That is, the patent document 2 discloses a method of producing a ceramic sintered body by densely compressing the ceramic powder by the pre-treatment such as hydrostatic pressure treatment and, then, elevating the temperature to 300 to 1500° C. by feeding a pulse current. However, the present inventors have confirmed that it is difficult to effectively prevent the formation of pores in the sintered body despite the aluminum nitride powder is pressed to possess a sufficiently increased density. Therefore, it is difficult to form a mirror surface having a high degree of smoothness on the aluminum nitride sintered body obtained by this method.